1. Field of the Invention
The present invention relates to a method for providing local anesthesia using liposomal encapsulated anesthetic and analgesic drugs.
2. Description of Related Art
Liposomes are lipid vesicles composed of membrane-like lipid layers surrounding aqueous compartments. Liposomes are widely used to encapsulate biologically active materials for a variety of purposes, e.g. they are used as drug carriers. Depending on the number of lipid layers, size, surface charge, lipid composition and methods of preparation various types of liposomes have been utilized.
Multilamellar lipid vesicles (MLV) were first described by Bangham, et al., (J. Mol. Biol. 13:238-252, 1965). A wide variety of phospholipids form MLV on hydration. MLV are composed of a number of bimolecular lamellar interspersed with an aqueous medium. The lipids or lipophilic substances are dissolved in an organic solvent. The solvent is removed under vacuum by rotary evaporation. The lipid residue forms a film on the wall of the container. An aqueous solution generally containing electrolytes and/or hydrophilic biologically active materials are added to the film. Agitation produces larger multilamellar vesicles. Small multilamellar vesicles can be prepared by sonication or sequential filtration through filters with decreasing pore size. Small unilamellar vesicles can be prepared by more extensive sonication. An improved method of encapsulating biologically active materials in multilamellar lipid vesicles is described in U.S. Pat. No. 4,485,054.
Unilamellar vesicles consist of a single spherical lipid bilayer entrapping aqueous solution. According to their size they are referred to as small unilamellar vesicles (SUV) with a diameter of 200 to 500 A; and large unilamellar vesicles (LUV) with a diameter of 1000 to 10,000 A. The small lipid vesicles are restricted in terms of the aqueous space for encapsulation, and thus they have a very low encapsulation efficiency for water soluble biologically active components. The large unilamellar vesicles, on the other hand, encapsulate a high percentage of the initial aqueous phase and thus they can have a high encapsulation efficiency. Several techniques to make unilamellar vesicles have been reported. The sonication of an aqueous dispersion of phospholipid results in microvesicles (SUV) consisting of bilayer or phospholipid surrounding an aqueous space (Papahadjopoulos and Miller, Biochem. Biophys. Acta., 135: 624-238, 1968). In another technique (U.S. Pat. No. 4,089,801) a mixture of a lipid, an aqueous solution of the material to be encapsulated, and a liquid which is insoluble in water, is subjected to ultrasonication, whereby liposome precursors (aqueous globules enclosed in a monomolecular lipid layer), are formed. The lipid vesicles then are prepared by combining the first dispersion of liposome precursors with a second aqueous medium containing amphiphilic compounds, and then subjecting the mixture to centrifugation, whereby the globules are forced through the monomolecular lipid layer, forming the biomolecular lipid layer characteristic of liposomes.
Alternate methods for the preparation of small unilamellar vesicles that avoid the need of sonication are the ethanol injection technique (S. Batzri and E. E. Korn, Biochem. Biophys. Acta. 298: 1015-1019, 1973) and the ether injection technique (D. Deamer and A. D. Bangham, Biochem. Ciophys. Acta. 443: 629-634, 1976). In these processes, the organic solution of lipids is rapidly injected into a buffer solution where it spontaneously forms liposomes--of the unilamellar type. The injection method is simple, rapid and gentle. However, it results in a relatively dilute preparation of liposomes and it provides low encapsulation efficiency. Another technique for making unilamellar vesicles is the so-called detergent removal method (H. G. Weder and O. Zumbuehl, in "Liposome Technology": ed. G. Gregoriadis, CRC Press Inc., Boca Raton, Florida, Vol. I, Ch. 7, pg. 79-107, 1984). In this process the lipids and additives are solubilized with detergents by agitation or sonication yielding defined micelles. The detergents then are removed by dialysis.
Multilamellar vesicles can be reduced both in size and in number of lamellae by extrusion through a small orifice under pressure, e.g., in a French press. The French press (Y. Barenholz; S. Amselem an D. Lichtenberg, FEBS Lett. 99: 210-214, 1979), extrusion is done at pressures of 20,000 lbs/in at low temperature. This is a simple, reproducible, nondestructive technique with relatively high encapsulation efficiency, however it requires multilamellar liposomes as a starting point, that could be altered to oligo- or unilamellar vesicles. Large unilamellar lipid vesicles (LUV) can be prepared by the reverse phase evaporation technique (U.S. Pat. No. 4,235,871, Papahadjopoulos). This technique consists of forming a water-in-oil emulsion of (a) the lipids in an organic solvent and (b) the substances to be encapsulated in an aqueous buffer solution. Removal of the organic solvent under reduced pressures produces a mixture which can then be converted to the lipid vesicles by agitation or by dispersion in an aqueous media.
U.S. Pat. No. 4,016,100, Suzuki et al., describes still another method of entrapping certain biologically active materials in unilamellar lipid vesicles by freezing an aqueous phospholipid dispersion of the biologically active materials and lipids. All the above liposomes, made prior to 1983, can be classified either as multilamellar or unilamallar lipid vesicles. A newer type of liposomes is referred to as multivesicular liposomes (S. Kim, M. S. Turker, E. Y. Chi, S. Sela and G. M. Martin, Biochim. Biophys. Acta. 728: 339-348, 1983). The multivesicular liposomes are spherical in shape and contain internal granular structures. A lipid bilayer forms the outermost membrane and the internal space is divided up into small compartments by bilayer septrum. This type of liposomes required the following composition: an amphiphatic lipid with net neutral charge, one with negative charge, cholesterol and a triacylglycerol. The aqueous phase containing the material to be encapsulated is added to the lipid phase which is dissolved in chloroform and diethyl ether, and a lipid-in-water emulsion is prepared as the first step in preparing multivesicular liposomes. Then a sucrose solution is shaken with the water-in-lipid emulsion; when the organic solvents are evaporated liposomes with multiple compartments are formed.
For a comprehensive review of types of liposome and methods for preparing them refer to a recent publication "Liposome Technology" Ed. by G. Gregoriadis., CRC Press Inc., Boca Raton, Florida, Vol. I, II, and III 1984.
Liposomes, vesicles of phospholipid membranes, have been studied in recent years as a way of altering the pharmacokinetic properties of encapsulated drugs. A few studies have focused on their potential as drug carriers in topical preparations, for example involving corticosteriods, econazole, progesterone and methotrexate. Liposomal formulations of these materials were found which when applied topically delivered more of these drugs into the skin than conventional vehicles, (enhanced penetration) while at the same time localizing their effect at the desired site of action (enhanced localization) (M. Mezei in "Liposomes as Drug Carriers" ed. G. Gregoriadis, John Wiley & Sons Ltd., New York 1988, pages 663-677).
Topical anesthetics are agents that reversibly block nerve conduction causing numbness and cessation of pain even after major stimuli. A topical analgesic agent is a substance which relieves pain without necessarily causing numbness, or which can relieve topical pain of a minor nature, but not of a great degree (Fed. Register 44, 69768-69866, 1979). These drugs are therefore used to treat or prevent pain. For operations of a peripheral or minor nature involving the skin, like removal of superficial skin lesions and plastic surgery, or intradermal allergen testing, split skin grafting, treatment of painful ulcers, venipuncture--the ideal way of anesthesia would be the topical application of local anesthetics.
The commercially available topical anesthetic preparations however, are not completely suitable for this purpose. Studies of Dalili and Adriani (Clin. Pharm. Ther., 12: 913-919, 1971) provided the first experimental evidence that manufactured preparations containing local anesthetics intended for use on the surface of the skin often lack a desired degree of efficacy. The preparations were tested on normal skin and on ultraviolet light burned skin for the ability to block itching and pricking induced by electrical stimulation. The only preparation judged sufficiently effective was one containing 20% benzocaine. But even the effect of this preparation disappeared within 60 seconds after it has been wiped off the test site. The authors indicated several possible reasons for the lack of efficacy, including the low concentration of the active ingredient, possible chemical change or interaction, for example, with other components and the penetration-preventing effect of the vehicle formulation used (J. Adriani and H. Dalili., Anesth., Analg. 50: 834-841, 1971).
At present, the most successful commercially available preparation for dermal anesthesia is a lidocaine-prilocaine cream, first reported by Juhlin et al. (Acta Derm Venereol. 59: 556-559, 1979). The cream consists of an emulsion containing 5% by weight of the eutectic mixture of lidocaine and prilocaine bases (EMLA) in water, thickened with Carbopol.RTM. (G. M. E. Ehrenstrom Reiz and SLA Reiz., Acta Anaesth. Scand., 26: 596-598, 1982). An application time of 60 minutes under occlusion achieves complete anesthesia to pin-pricks, and the anesthetic effect lasts one to two hours (H. Evers et al., Br. J. Anaesth., 58: 997-1005, 1985).
In general, to achieve adequate local anesthesia of the skin using known preparations, a relatively excessive amount of drug, a prolonged application period or invasive methods are required. For adequate surgical anesthesia, the local anesthetic must be injected subcutaneously in order to reach sensory nerve endings lying in the dermis. When injecting a local anesthetic, pain is produced by the needle's penetration and by the deposition of the anesthetic solution. Distortion of the wound or performing the infiltration of large areas can also be problems in surgical cases (L. Juhlin, H. Evers, and F. Broberg., Acta Derm. Venereol., 60: 544-546, 1980).
In contrast to anesthetizing the skin, anesthesia of mucous membrane covered surfaces can be produced by topical application of local anesthetics quickly and easily. Unfortunately, rapid absorption of the local anesthetic through these surfaces into the circulatory system may reduce the duration of local anesthetic action, and since these drugs have low therapeutic ratios, may possibly cause systemic toxicity (J. A. Wildsmith, A. P. Rubin and D. B. Scott., Clin. Anaesth., 4: 527-537, 1986).
Thus, there is a continuing need in the art of local anesthesia for a preparation that is safe, yet effective on either unbroken skin, or on mucous membranes which provides a proper rate of drug permeation without discomfort or a risk of systemic reactions.
Local anesthetic agents previously have been encapsulated into liposomes. (Papahadjopoulos et al., Biochim. Biophys Acta, 394: 504-519, 1975). However, the liposome encapsulated local anesthetic was not used for producing local anesthesia or analgesia but rather was prepared as a way of studying the drug's mechanism of action, i.e. the interaction of the local anesthetic with the phospholipid bilayers, which in effect served as a model for a cellular membrane.